Although most of the visible light from H+ regions is in the form emission lines, the same is not true at longer wavelengths, where most of the energy is radiated in a continuum. There are, in fact two continua: bremsstrahlung or free-free emission from the ionized plasma, which causes the radio flux, and thermal radiation from dust grains, which dominates the infrared region shortward of about 3-mm wavelength

Free-free emission occurs as a result of random encounters between electrons and ions. As an electron moves through the plasma, it experiences ever-changing accelerations due to electrical forces between it and the charged ions. Every burst of acceleration generates electromagnetic radiation over a wide range of frequencies. The term free-free refers to the fact that, as opposed to the situation in an ionization or recombination, the electron is not attached to an atom either before or after the encounter. The net result of all these collisions is a continuous band of almost uniform emission at radio and infrared wavelengths. At low frequencies (below perhaps 1 GHz, depending on the H+ region), the emission decreases due to re absorption of the free-free radiation by the gas, so radio astronomers prefer to study H+ regions at high radio frequencies if possible. Modern aperture synthesis radio telescopes can produce pictures of the radio emission from an H+ region which have almost as much detail as an optical photograph , but which are completely free from interstellar extinction. In many cases the radio emission is the only information we have about H regions, since a large fraction of these objects, especially those on the far side of the Galaxy, are obscured at visible wavelengths. For optically visible H+ regions, a comparison of the radio and infrared continua with the visible hydrogen recombination lines can provide a reliable guide to the amount of dust in front of the object.

Almost all H+ regions show a large infrared continuum at wavelengths in the range 3-3000 ?m. The emission arises from small dust grains at temperatures between 30 and 300K. Some of the dust grains exist within the ionized regions and become heated by absorbing radiation from the ionizing stars and from the ultra¬violet recombination lines in the nebula. Other, cooler dust grains exist in the neutral gas that surrounds many H+ regions, and are heated by the light that emerges through the ionization front. Many H~ regions have so much dust associated with them that essentially all the risible and ultraviolet light from the ionizing stars is converted to infrared radiation. H+ regions are therefore among the most powerful sources of infrared .radiation in our Galaxy, and have luminosities of up to 107LO. In some cases, how¬ever, including the Orion Nebula, some of the infrared emission may come from molecular clouds very close to the H+ region rather than from the immediate vicinity of the ionized gas. Weak continuum emission is also seen in the visible spectrum of H+ regions. The emission is due partly to starlight scattered off the dust grains in the nebula and partly to atomic processes in the plasma.

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